Dehydration in cattle occurs when the amount of bodily fluids is reduced from normal. Dehydration can cause hypovolemic shock and electrolyte imbalances. By far the most common conditions that cause dehydration in dairy animals are endotoxic mastitis in cows and diarrhea in calves. In fact, in both of those conditions, dehydration and accompanying shock may be one of the most common causes of death in affected animals. Correcting dehydration is thus critically important. Choosing the right fluids, the amount to give and the method to administer are also important.
    In adults and calves, dehydration can be diagnosed by degree of enopthalamus, or sinking of the eyeballs. When the lower lid is pulled away from the eyeball there should not be a space between the globe and the socket. In dehydrated animals, there will be a visible space. The greater the space the greater the dehydration. Checking the degree of skin tenting is another technique. One twists the skin and counts the seconds until the skin returns to normal. Over 2-3 seconds indicates dehydration. Most people with significant cow experience can probably diagnose dehydration by looking at the face and eyes. However, by any method, most observers will only be able to diagnose dehydration of 4%-5% or greater.
    The two most important goals of fluid therapy are to replace fluid volume in the blood and body tissues, and to correct acid/base imbalances. Volume replacement in cattle almost always requires fluids containing electrolytes such as sodium, potassium and chloride. The total concentration should be equal or greater to blood plasma, otherwise the fluids will not remain in the bloodstream. This is why giving water intravenously, or water with 5% dextrose, does not work to correct dehydration. Volume can also be replaced by using a combination of hypertonic, or more concentrated, fluids IV and hypotonic, or less concentrated fluids orally. Hypertonic fluids must be given IV though, because they are irritating to tissues. If given subcutaneously, sloughing of the skin can occur.
    Acid/base imbalances can be divided into acidosis and alkalosis. Dehydrated adult cows are rarely acidotic while dehydrated calves are rarely alkalotic. This is important in choosing the correct replacement fluids. Thus for cows, non-alkalizing fluids like saline are appropriate, while sodium bicarbonate would be more appropriate for calves. Calves with diarrhea that are 1 week or less in age are less likely to be acidotic than older calves; however, they will not be alkalotic so using sodium bicarbonate is appropriate. Lactated ringers can also be used in calves; it is mildly alkalizing, so it is best used for very young calves or calves that have a strong suckle response. The suckle response is fairly well correlated with acid/base status in calves, so if you put your finger in the mouth and the calf sucks strongly, it is not likely acidotic. An important point is that acidotic calves given IV fluids that are not alkalizing do not correct the acidosis on their own. Oral fluids can be used to correct acidosis, and good quality electrolytes designed to treat calf diarrhea will have alkalizing agents in them. Beware though; there are plenty of calf electrolyte products on the market that are not correctly formulated and should not be used. Also, severely acidotic calves with poor suckle response may not absorb oral fluids quickly or effectively. Oral electrolytes can be given before or after a milk feeding, or can be fed free choice throughout the day.
    Most people underestimate how much fluids dehydrated animals need. It is often difficult to diagnose less than 5% dehydration. A 40 kg calf that is 5% dehydrated is short at least 2 liters of fluid. Since scouring calves are constantly losing fluids, and since there are normal maintenance requirements, no dehydrated, scouring calf should ever get less than 2 liters of fluid. A good rule of thumb is at least 4 liters, or roughly 1 gallon should be given in total, of IV and oral fluids within the first few hours, and followed by at least 2 liters of oral fluids at the next feeding. In severe diarrhea cases, calves can lose up to two gallons of fluids in 24 hours. Most dehydrated calves can tolerate 3 liters of IV fluids given rapidly through a 14-gauge catheter or needle. If one cannot manage to place a needle or catheter IV, isotonic fluids can be given subcutaneously or intraperitoneally. Almost anyone can be trained fairly quickly by a veterinarian to administer SQ or IP fluids. A 1,500 pound cow that is 5% dehydrated is 23 liters, or about 6 gallons, low on fluid. Thus, any cow with detectable dehydration needs at least 10 gallons of fluids. The simplest way to do this is to use hypertonic saline IV followed by water orally. Cows that drink well within 10 minutes of hypertonic fluid administration usually do not need to be pumped with more water.
    Any dairy farm will have cattle or calves that suffer from dehydration. Fluid therapy is a critical treatment and can save lives. The principles are pretty simple but important. Ask your veterinarian to design fluid therapy protocols for your farm.
    Information for this article came from Smith, Fluid Therapy in Calves, and Roussel, Fluid Therapy in Mature Cattle, Veterinary Clinics of North America, 2014.
    Bennett is one of four dairy veterinarians at Northern Valley Dairy Production Medicine Center in Plainview, Minnesota. He also consults on dairy farms in other states. He and his wife, Pam, have four children. Jim can be reached at bennettnvac@gmail.com with comments or questions.